Headasteam is a large Yo-kai whose body is mostly compossed by his head, gray in color, with the mouth area colored red; sporting four large fangs inside the mouth. He has three eyes: a green eye in his front, and two uneven eyes resembling pressure meters, the left one being larger and more detailed -with scale-like markings rounding it-. He has comparatively shorter and smaller arms with gloved hands. He sports a driver's hat, alluding to his position in the Hexpress.
Headasteam appears to have a very short temper.
He has enough energy to keep the Hexpress running, being connected to the engine system.
In this boss battle, each blow Headasteam receives fills a two-fold red bar in the left side of the background and a blue bar in the right side of the background. Pinning the valves in said side causes each of the bars to fill faster.
Headasteam appears at the engine room of the Yokai Hexpress.
""Boso" describes the following: "running wildly; reckless driving; runaway; rampage".
Headasteam is based on the idiom "a head of steam", which describes a situation in which a person or an activity starts to become very active or successful.
Chomino comes from "Chaud" (hot) and "Cheminot" (railroader)